BACKGROUND TECHNICS Building material is in short supply in a large number of countries and above all building material that is available in each country and that may be manufactured in the country at low costs. Inexpensive labour is available whereas investments in the form of machines generally are imported.

Traditional building materials that are available are cement, brick, reinforcing iron etc.

Reinforcing iron for example often is far too expensive to use, as is a variety of insulation.

Gypsum wallboards, wood-wool slabs etc. have other disadvantages because of the difficulties in arranging screws in the board without having arranged some reinforcement in the form of pegs etc. Traditional construction materials, for example of gypsum board, fibreboard, wood-wool slabs or particle boards call for large investments in machine equipment. In manufacture of particleboard and wood-wool slabs large mechanical presses are demanded to compress chips or wood-wool.

Factors to affect the shaping of the building are the climate, as for example high relative humidity, hurricanes and also vermin as termites, grasshoppers etc.

Also the risks of fires and the lack of rational fire prevention have to be taken into consideration in building projects.

Cement and vegetable fibre are materials available in most countries at very low costs. In many

countries vegetable fibres are practically almost free of costs. Cement is a reliable building material that in itself not contributes in the emergence of allergies etc.

Wheat is one of the cereals grown most in the world, but the number of straw types is large, in South Africa for example are at least about thirty different kinds.

In countries like South Africa and others is an acute need of houses, the houses have to be cheap to produce and the rate of production has to be high. And there are traditions concerning the arrangement of the exterior front face.

DESCRIPTION OF THE INVENTION The object of the present invention is to achieve a building method and techniques belonging to it that admits building at low costs and that solves the above mentioned problems. The object is achieved with measures evident from the claims.

The method according to the invention implies that in manufacture of a wall section for a main body of a house, the wall section comprising an insulation slab with a surface section, a thin reinforced slab is placed as a wall facing in which the slab is built up of three layers of cement mixture with a first layer containing a cement mixture A, a second layer containing a cement mixture B with reinforcement of vegetable fibres and a third layer containing a cement mixture C. Each layer of cement mixture A, B and C is applied by spraying or manual throwing by means of ordinary implements. The first layer is given a thickness within the interval of 1 and 3 mm. Onto the surface of the first layer a second layer of cement mixture B is applied. The second layer is applied in a number of crusts and every crust has a distinguishing cement mixture B, Bl etc. The distinction of each cement mixture Bl, B2, B3... Bn is characterized in that the cement mixture Bl of the first crust contains a larger part of cement than the cement mixture B2 of the second crust. The cement mixture of the following crusts

B3.. Bn contains a lower part of cement than the cement mixture B2 and so on of the preceding crusts. Onto the surface of the second layer a third cement mixture C is applied.

When spraying slurry a"concrete pump"is used that makes it possible to control the amount of slurry per unit of time. The cement mixture is transported from a container to a nozzle and sprayed from a nozzle onto the wall section by means of compressed air or for example a mechanically rotating device attached to the nozzle. The mechanically rotating device shows an axle with radial attached blades.

In spraying the crusts of the second layer the cement mixture has to be sprayed onto the surface with such a great speed,-force-, that the major part of air holes is squeezed out of the cement mixture. Air holes decrease the strengths of the slab. The remaining air holes are advantageously removed by means of a stripper, for example a ruler that is drawn across the cement mixture and evens out the thickness of the mixture. By squeezing out the air the layer gets greater strength.

Vegetable fibres as for example straw, but also other types of vegetable fibres as for example bamboo, coconut fibre, shavings from wood, may be used. All fibres have to be dried before use and contribute to reinforce the slab. Slabs with reinforcement may be exposed for greater bearing pressure than slabs without reinforcement. In a slab with reinforcement the reinforcement will contribute to a decrease of the crack risk and that emerging cracks will not spread further, i. e. propagate, in the slab.

Moreover, the vegetable fibres contribute to make it easy to use screws and nails in the slab without the need of different types of plastic plugs or the like.

In manufacture of wall elements or a wall section comprising a framework the latter firstly is clad with insulating slabs. Into the surface of the insulation longitudinal slots are cut for a strip of inorganic

material with a preferable thickness of 4 mm. The width of the strip is adjusted to the thickness of the insulation and is always smaller than the thickness of the insulation. The strip protrudes about 1 to 3 mm from the surface of the insulating slab. Onto the surface of the insulating slab and the protruding edges of the strips is sprayed or thrown a first layer of cement mixture A up to a thickness of about 1 to 3 mm. Totally three layers of cement mixture A, B, C are applied onto the insulating slab. Onto he surface of the first layer, cement mixture A, is sprayed or thrown a second layer of cement mixture B with a number of crusts. The content of vegetable fibres in the second layer constitutes the reinforcement of the slab. The composition of the crusts changes since the part of cement in the mixture decreases gradually outwards. The reason for this change is to get a slab that partly has a high rigidity and partly to minimize the risk of propagation of emerged cracks. The thickness of the second layer is 3 to 40 mm, preferably about 5 to 8 mm. Onto the surface of the second layer is sprayed a third layer, cement mixture C, with a thickness of about 5 mm.

The framework is manufactured preferably of cylinder-shaped thin metal profiles of zinc-coated steel sheet, for example in a thickness of 0,4 to 2,5 mm, that has formed into U-, L-, Z-shape. Clenching, bolting or any other similar method assembles the framework. An alternative framework is a nailed construction with wooden battens.

The cement mixture A in the first layer contains preferably one part of cement and one part of sand elutriated in water. Other filling materials may also be used as for example crushed glass, slag and inorganic colouring pigments. The first layer even may contain vegetable fibre. In reconditioning existing wall surfaces with plaster existing plaster may be used as a first layer.

The cement mixtures Bl, B2... Bn, in the second layer contain pre-treated vegetable fibre, preferably wheat straw, crushed and cut into lengths of about 0,5 to 2 cm and serves as reinforcement of the slab. The length of the vegetable fibre is limited by the possibility to get the slurry through the gun device. It is possible to use vegetable fibre within an interval of 0,5 to 70 mm.

The cement mixture Bl in the first crust of the second layer contains preferably two parts of vegetable fibre (straw), two parts of sand, two parts of cement and water up to a concentration that is adapted to an application by spraying. Then a second crust is applied that shows a lower content of cement concentration than the first crust. The cement mixture in every following crust contains a lower cement concentration than the cement mixture of the preceding crust. The number of crusts sprayed in the second layer is preferably within an interval of 2 to 6 pieces.

The cement mixture C in the third layer contains preferably six parts of sand, one part of cement and water. As filling material even other inorganic minerals or pigmented slag may be used in a concentration that is adapted for pumping, i. e. slurry, for application onto a wall section by spraying or throwing.

Pre-preparation of vegetable fibre, for instance straw, is done by soaking the dry, cut and crushed fibre in hot, not boiling, water up to 18 hours depending on the straw quality. In soaking the surface of the straw is cleansed from its waxen layer and from certain kinds of sugar. Furthermore, the fibre stays mouldable. After the soaking the vegetable fibre is ought to be of a consistency that easily may be mixed to slurry to be pumped or sprayed.

DESCRIPTION TO THE DRAWINGS The invention is described subsequently as an example of embodiments in connection with the enclosed drawings.

Figure 1 shows the front side of a house.

Figure 2 shows a side view of a house.

Figure 3 shows a wall module including a framework.

Figure 4 shows a section along line A-A in figure 3.

Figure 5 shows an enlarging of a part in fig. 4.

Figure 6 shows a section in a building slab with the different layers of cement mixtures.

Figure 7 shows a detail of a building slab including transverse strips.

In figure 1 are shown wall sections 1,2 in a small house which wall sections 1,2 may be manufactured with advantage in industrial scale and then mounted on a base plate 3.

In figures 3,4 is shown a wall element 4 comprising a framework 5 with bars 6 and one at the outside 7 of the framework 5 arranged insulation 8. The bars 6 may be of metal profile or woodwork. Bars 6 of metal are used with advantage when then climate is humid or where there is a risk of affects from termites or other vermin.

The insulation 8 has the function to be insulating and to be the frame for mounting the exterior wall slab 9. An example of insulation 8 is expanded polystyrene, for instance Styrofoam, the thickness of the insulation 8 being adapted to the climate etc. A thinner slab may be used for houses in South Africa whereas a thicker insulation 8 is needed for buildings in Sweden.

The insulation 8 is nailed, screwed on or in some other way arranged to the framework 5. The framework 5 together with the insulation 8 constitutes a wall element 4. In the surface section 10 of the insulation 8 longitudinal slots 11 are arranged preferably perpendicular to the surface 10. In the slots 11 strips 12 of for instance thin, about 4-mm thick cement flakes are arranged. The strips 12 protrude one or some

millimetres, for instance about 1 to 3, above the surface 10 of the insulation 8.

At the surface 10 of the insulation an exterior wall facing in the shape of the wall element 13 is arranged. The wall facing 13 comprises strips 12 and a wall slab 9. The strips 12 are fixedly arranged to the wall slab 9. Fig. 5 shows an enlarging of a wall slab 9 with strips 12 and insulation 8. The wall slab 9 is built up of three layers 14,15,16 of the cement mixtures A, B, C. Fig. 6 shows separately the strips 12 and the wall slab 9. By arranging the strips 12 at the slab 9 a more rigid wall facing 13 is attained that is able to take up larger strain than a single wall slab 9.

The first layer 14 of the cement mixture A contains one part of cement and one part of sand that is mixed with water to a slurry that is applied by spraying onto the surface 10 of the insulation 8 to a thickness within the interval of about 1 to 3 mm. The cement mixture A may even contain other filling materials such as differently coloured minerals, slag, crushed glass and also vegetable fibre. The first layer 14 works as a primer for the following layers of cement mixture.

The second layer 15 of the cement mixture B is applied in a number of crusts 2,15.3... 15. n, preferably two to six crusts. The cement mixture B contains reinforcing of vegetable fibre, for instance from straw but also bamboo, coconut fibre, shavings that are crushed and cut into lengths of about 0,5 to 2 cm.

The cement mixture B contains preferably two parts of straw, two parts of filling material, for instance sand, two parts of cement that is mixed with water to a slurry for application by spraying or throwing onto the wall section.

The second layers 15 first crust 15.1 with a cement mixture B is applied at the surface 17 of the first layer 14. Then a second crust 15.2 with cement mixture B1 is applied to the first crust with a reduced

part of cement. In every following crust 15.3... 15. n of cement mixture B2, B3... Bn the part of cement decreases gradually. The thickness of the second layer 15 is within the interval of about 5 to 8 mm.

In preparing the cement mixture B the cut into pieces and crushed, dry vegetable fibre, for instance straw, is soaked in hot water, not boiling, up to 18 hours. The soaking duration depends totally on the straw quality. The vegetable fibre has to be soaked for so long time that the straw which is an integral part of the slurry not gets stuck in the nozzle of the gun device.

In applying the cement mixture B it has to be sprayed or thrown onto the surface with such a great speed and force that the major part of the air holes is squeezed from the slurry. The strength of the second layer 15 decreases if the amount of air holes increases.

Remaining air holes are removed by planation with a stripper, for instance a ruler.

The cement mixture C of the third layer 16 contains one part of cement, six parts of filling material, for instance sand, mixed with water to a slurry for application by spraying and is applied onto the surface 18 of the second layer 15. The filling material may also consist of different kinds of minerals, slag, crushed glass and so on. The thickness of the third layer is up to about 5 mm.

In manufacturing wall element 4 bars 6 are arranged in a fixture and attached to a framework 5 by means of for instance a bolt connection or with another means of attachment. Then an insulation slab 8 is fixed to the outside of the framework 5. In the surface 10 of the insulation 8 longitudinal slots 11 are cut and in the slots are arranged strips 12 of for instance thin cement flakes.

A first layer 14 of cement mixture A is applied onto the surface 10 by spraying a slurry of cement mixture A under high pressure. This first layer 14 will

function as a primer for the next layer 15 of cement mixture B.

The now manufactured wall elements 4 provided with a first layer 14 of cement mixture A then are erected upon the base plate 3. After erection of all wall elements 4 firstly the spraying of cement mixture B takes place and then of cement mixture C onto the exterior surface of wall element 4.

Of course the invention is not limited to the method of applying layers 14,15,16 of cement mixture A, B, C onto the wall elements 4 but the method may be used with advantage in the manufacture of slabs. Figure 7 shows a single slab 9. In manufacturing slabs, frameworks with bottom boards and edges are used with advantage.

Before spraying the first layer 14 of cement mixture A onto the bottom of the framework, the surface of the bottom has to be treated with some medium to avoid adherence between the layer 14 and the bottom board. Then the layers 15 and 16 are sprayed into the framework. The slab may be used in the same way as a gypsum board and as covering material for walls and ceilings. The invention may be used in renovation of existing houses and their wall sections, where the existing plaster replaces the first layer A.